Iron nitride (γ′-Fe4N and ε-Fe3N) magnetic nanoparticles were successfully fabricated via gas nitriding method using Fe2O3 nanoparticles under NH3/N2 atmosphere conditions. The core-shell structure exhibited with the core of ε-Fe3N and the shell of γ′-Fe4N. A systematically investigation was also made to explore the effects of varying nitriding durations on the microstructures and magnetic properties of iron nitride nanoparticles. The results show that as the nitriding durations increased, the grain size keeps increasing. Moreover, as the content of the γ′-Fe4N phase increased, the saturation magnetization (Ms) was enhanced, whereas, there was a reduction in the coercivity (Hc). Notably, the iron nitride nanoparticle prepared after 3 h exhibits excellent soft magnetic properties with high Ms up to 87 emu/g.